Sandor Katz is the author of Wild Fermentation and lectures extensively on topics related to fermentation.

Courtesy of the author

The list of fermented food in our lives is staggering: bread, coffee, pickles, beer, cheese, yogurt and soy sauce are all transformed at some point during their production process by microscopic organisms that extend their usefulness and enhance their flavors.

The process of fermenting our food isn't a new one: Evidence indicates that early civilizations were making wine and beer between 7,000 and 8,000 years ago — and bread even before that.

But was exactly is fermentation? And how does it work? Those were the questions that fascinated Sandor Katz for years. Katz calls himself a "fermentation revivalist" and has spent the past decade teaching workshops around the country on the ancient practice of fermenting food.

Katz collects many of his recipes and techniques in a new book, The Art of Fermentation, in which he describes fermentation as "the flavorful space between fresh and rotten."

"If you walk into a gourmet food store and start thinking about the nature of the foods that we elevate on the gourmet pedestal, almost all of them are the products of fermentation," he tells Fresh Air's Terry Gross. "Fermentation creates strong flavors. But they're not always flavors that everybody can agree on."

Take cheese, for instance. Cheese exists in a variety of flavors, including the extra-stinky varieties Katz says he fancies. "But once in a while I'll buy cheese and I've learned that some friends will smell the cheese and walk out of the room," he says. "They'll never think about putting that in their mouths. ... So around the world, you find these iconic foods created by fermentation that create strong, strong flavors that become strong markers of cultural identity and in many cases, people who have not been raised within the culture find these foods very challenging."

In addition to enhancing flavors, fermentation also allows food items to be preserved well past their shelf-life date, says Katz.

"It's not forever like canned foods that you can put into a pantry or storm cellar and forget about for 10 years and still eat it," he says. "These foods are alive, they're dynamic, but they're extremely effective strategies for preserving food through a few seasons, which is really the point."

Starting With Sauerkraut

For fermentation newbies, Katz recommends starting with sauerkraut because it's particularly easy to make. To begin, take a cabbage and any additional vegetables you want and chop it up. Put your chopped veggies in a large bowl and lightly salt them. (Katz notes that he never measures the salt because there's really no "magic number for how much salt to use.")

After salting the veggies, which helps get rid of excess water, Katz squeezes them for a few minutes to release their juices, so that they can be submerged under their own liquid. (Katz says he hardly ever adds water to his kraut, because the flavor is more concentrated if you use only the vegetable juice.) He then stuffs the veggies and the juices they've released into a jar.

"You want to press really hard to force out any air bubbles," he notes. "And you want to make sure that the vegetables are pressed down under their juices. And then just seal the jar — but be aware that pressure will be produced, so you don't want to leave it for days and days."

Katz recommends checking the jar on a daily basis to release the pressure — and then after maybe 3-5 days, enjoying your new creation.

"The flavors transform very quickly," he says. "The bacteria proliferate, the texture changes, and what I recommend to people experimenting for the first time, is just to taste it at periodic intervals. And then you're getting a sense of whether you're liking it more and more as the flavor gets more acidic or whether it's acidic enough and you want to move it into your fermentation-slowing device, which is your refrigerator."

Once you've mastered the simple kraut, Katz says you can add spices and/or other items like apples or cranberries to your jar. "You can basically use any season you like," he says.

Interview Highlights

On making yogurt

"The principle behind yogurt is almost the same as the principle behind sauerkraut. We're using lactic acid bacteria to preserve food. The method for it is somewhat different. There are many different types of lactic acid bacteria. And the ones that are used in most yogurt traditions are the ones we would describe as thermophilic — meaning they are most active in an elevated temperature range. So usually when you make yogurt, you want to incubate the yogurt by creating an environment that stays in between 110 degrees and 115 degrees Fahrenheit.

"Yogurt is often the classic example of what is called a cultured food. And the cultures are the community of bacteria that you're introducing and the act of introducing it is called culturing the food. So yes, to make yogurt, you always need a batch of mature yogurt and that's what you introduce — a spoonful of mature yogurt that you want to turn into fresh yogurt."

On probiotics

"Yogurt is a fermented food, and many different types of fermented food — particularly those fermented by lactic acid bacteria — can be thought of as probiotics. ... [That can include] fermented vegetables and not only yogurt but kefir and many fermented dairy products and a large group of beverages that I really enjoy that I would group together as sour-tonic beverages. Right now the most famous example of that in the United States might be kombucha. ... What's probiotic about these foods is that the lactic acid in them can help to replenish and diversify the populations in our gut, which due to a number of factors in our contemporary lives — including antibiotic drugs, antibacterial cleansing products, chlorine in water — are subjected to more or less constant attack."

On why good bacteria are beneficial

"Bacteria in our gut enable us to live. We could not survive without bacteria. ... They allow us to digest food, to assimilate the nutrients in our food; and they play a huge role, just beginning to be understood, in our immune functioning and in many other processes in our bodies. All life has evolved from bacteria and no other form of life has lived without bacteria. ... Our bacteria perform all sorts of essential functions for us, and because we are continually attacking them effectively with all of these chemicals in our lives, simply replenishing and diversifying these populations has a benefit for us."

On making sour pickles

"To ferment sour pickles, you take small cucumbers and you mix up a brine, which is simply salty water. The strength of the brine has implications. Usually I'll add grape leaves as a means to help keep the cucumbers crunchier longer. And then lots of dill and lots of garlic and then it's just a matter of waiting a couple of days to a couple of weeks, depending on the temperature. The metabolism of all of these organisms speeds up in warmer weather, so in summer heat, the process goes faster. ... A cellar is really best if you're looking to preserve sour pickles for any length of time. Because the cucumbers will have a tendency to float to the surface, I'll usually place a plate on them to keep them weighted down below the surface of the water."

On the space between rotten and fresh food

"We reject certain food because it is rotten. Certain food we can see is fresh. But there is this creative space between fresh food and rotten food where most of human culture's most prized delicacies and culinary achievements exist."

This is FRESH AIR. I'm Terry Gross. Yogurt, pickles, cheese, sour cream, salami, vinegar, sauerkraut, soy sauce, breads, chocolate, coffee, wine and beer are just a few of the most popular fermented foods and beverages. My guest, Sandor Katz, a self-described fermentation fetishist, has spent nearly two decades exploring the realm of fermentation and learning about the digestive benefits of bacteria present in living fermented foods.

Katz has written a new book called "The Art of Fermentation." It has a forward by Michael Pollan. The book is about the hows and whys of fermentation with advice about how to make fermented foods at home. Katz grew up in Manhattan but now lives in Tennessee, where he has a garden and ferments many of the vegetables he grows. He's taught hundreds of fermentation workshops across America.

Sandor Katz, welcome to FRESH AIR. I think the biggest surprise to me in your book was thinking of fermented foods as probiotic foods. To me, probiotic foods, that's yogurt. I never think of, like, sauerkraut or other fermented foods as being probiotic.

SANDOR KATZ: Well, yogurt is a fermented foods, and, you know, many different types of fermented foods, particularly those fermented by lactic acid bacteria, can be thought of as probiotics.

GROSS: So what other foods does that include?

KATZ: Well, fermented vegetables, not only yogurt but kefir and many other types of fermented dairy products and, you know, a large group of beverages might be kombucha, but there's kavas, there's mauby, there's tepache.

GROSS: I have no idea what you're talking about.

(LAUGHTER)

KATZ: Well, I'm giving you names of beverages that people around the world drink. I mean these lightly fermented beverages, you know, of which, you know, kombucha has burst upon the American scene, you know, are popular in the places that they come from. And so this is a pattern of fermentation seen around the world, these lightly fermented beverages that contain live lactic acid bacteria, which really are very beneficial and stimulating for human beings.

GROSS: So getting back to the probiotic qualities, the good-guy bacteria qualities of fermented foods, something like sauerkraut, I've always thought of that as being kind of vinegary and acidic and therefore not great for my digestive system, but are you saying that it has probiotic qualities?

KATZ: Yeah, sure. I mean, sauerkraut does not have vinegar in it, at all. Vinegar is an acid that is acetic acid. The acid in sauerkraut is lactic acid, and this is the same acid that's found in yogurt. And, you know, almost all of the probiotic foods are probiotic by virtue of the presence of lactic acid bacteria, and they produce as their metabolic byproduct lactic acid. So that's, you know, a characteristic of, you know, almost all probiotic foods.

GROSS: So while we're on the subject, you recommend eating fermented foods, you know, on a regular basis but not a lot at a time.

KATZ: Yeah, in general these foods have been used as condiments, as embellishments of food. And, you know, every spoonful has billions of bacteria, and it's not really a matter of eating huge quantities of it. It's a matter of eating them pretty regularly. And really what's probiotic about these foods, you know, is that the lactic acid bacteria in them can help to replenish and diversify the populations in our gut, which due to a number of chemical factors in our contemporary lives, including antibiotic drugs, antibacterial cleansing products, chlorine in water, are subjected to more or less constant attack.

So for us in the 21st century, more than for people in the past, we need to consciously thinking about replenishing and diversifying these populations.

GROSS: And just briefly, what do the good-guy bacteria do in your gut?

KATZ: Well, bacteria in our gut enable us to live. You know, we could not survive without bacteria. We've all grown up, you know, indoctrinated by what I call the war on bacteria and this idea that bacteria in general are dangerous and bad for us, and our lives would somehow be better or safer if we could, you know, eradicate them.

But, you know, in fact we could not live without bacteria. They enable us to digest food, they enable us to assimilate the nutrients in our food, and they play a huge role, just beginning to be understood, in our immune functioning and in many other regulatory processes in our bodies.

I mean, remember, all life has evolved from bacteria, and no other form of life has lived without bacteria. So our bacteria perform all sorts of essential functions for us, and because we are continually attacking them effectively with, you know, all of these chemicals in our lives, you know, we simply, you know, replenishing and diversifying these populations has a benefit for us.

GROSS: I love the way you describe fermentation as a flavorful space between fresh and rotten.

(LAUGHTER)

GROSS: Do you want to describe what you mean?

KATZ: Sure. I mean, I think that one of the most, you know, basic lessons that we learn as, you know, small children from our parents is, you know, what is appropriate to put in our mouths. We reject certain food because it is rotten. You know, certain food we can see is fresh.

But there is this creative space between fresh food and rotten food where, you know, most of human culture's, you know, most prized, you know, delicacies and culinary achievements exist in this space. If you walk into a gourmet food store and start thinking about the nature of the foods that we, you know, elevate on the gourmet pedestal, almost all of them are the products of fermentation.

Fermentation creates strong flavors. They're not always flavors that everybody can agree on. I think cheese illustrates this really well. So, you know, around the world you find these iconic foods produced by fermentation that create strong, strong flavors that become really markers of cultural identity, and in many cases people who have not been raised within the culture, you know, find these foods, you know, very challenging.

GROSS: So what are the fermented foods that are part of your cultural identity?

KATZ: Well, growing up in New York City, one of my very favorite foods were sour pickles, what some people might know as kosher dills, and these are pickles that are made without any vinegar, simply in a brine, and the acidification comes from fermentation.

And I feel like I was just imprinted with this flavor of lactic acid from a very young age. You know, this was just a flavor that I really, really loved.

GROSS: So tell us how the kind of pickle you love is made.

KATZ: To ferment sour pickles, you take small cucumbers, and you mix up a brine, which is simply salty water. The strength of the brine has implications, you know, usually a 5 percent brine, which means in relation to the weight of the water, 5 percent salt, and usually I'll add grape leaves as a means of helping to keep the cucumbers crunchier longer.

And then lots of dill and lots of garlic, and then it's just a matter of waiting, you know, from a couple of days to a couple of weeks, depending on the temperature. The metabolism of all of these fermenting organisms speeds up in warmer weather. So in summer heat, which is when cucumbers tend to be ripe, the process goes faster. So it might be as few as, you know, three or four days.

In more moderate temperatures, or if you have a cellar, it could be a couple of weeks.

GROSS: So you just let it sit out, or do you have to put in an incubator?

KATZ: No incubator, just at room temperatures and actually the cooler the better. You know, a cellar is really best if you're looking to preserve sour pickles for any length of time, but you can really do it at any ambient temperatures, as long as you understand that it'll go faster when the temperatures are warmer.

Because the cucumbers will have a tendency to float to the surface, where they can be exposed to oxygen, which can promote the growth of yeasts and molds, I'll usually place a plate on them to keep them weighted down below the surface of the water.

GROSS: One of the reasons for fermenting food is that it lasts longer. Like, pickles will last longer than cucumbers will if just left out. In fact, like, when you buy a pickle, it's usually in a pickle barrel. It's just, like, not sitting in a refrigerator, it's just sitting in a pickle barrel. How long will it last?

KATZ: Well, fermentation is really the classic means of preserving, especially in relation to vegetables, but in relation to many other types of foods, as well. And, I mean, actually I am nearing the bottom of a barrel of radish kraut that I made at the beginning of November. So that has just passed its seven-month mark, not in any refrigeration, just in a cellar, and it is wonderful.

You know, it really - once it reaches a certain stage of acidity in a cool environment and in a salty environment, it pretty much plateaus, and for a very long time, you can eat it. It's not forever. It's not the way we think of, you know, canned foods that you can put into a pantry or storm cellar and forget about for 10 years and still eat. I mean, these foods are alive, they're dynamic, but they're extremely effective strategies for preserving food through a few seasons, which is really the point.

I mean, you're taking your harvest at the beginning of the season, when you won't have fresh vegetables, and you're preserving it through the season where there are no fresh vegetables, and it's easy to make it last, you know, until there are fresh vegetables again.

GROSS: If you're just joining us, my guest is Sandor Katz. He's the author of the new book "The Art of Fermentation." And we'll talk more about fermentation after we take a short break. This is FRESH AIR.

(SOUNDBITE OF MUSIC)

GROSS: If you're just joining us, my guest is Sandor Katz. He's the author of the new book "The Art of Fermentation." So you suggest that if you want to start making fermented foods that a good place to start is sauerkraut because it's really easy. What's your recipe for making sauerkraut?

KATZ: Well, sauerkraut is incredibly easy to make. It's fast, you don't need any special equipment, you don't need any special cultures, and it is intrinsically safe. So you take a cabbage or a combination of vegetables or really any vegetable you want, and the first thing is to create surface area. So you chop the vegetables or grate the vegetables, and then in a large bowl, lightly salt them. There's no magic number for how much salt to use.

Most of the commercial manufacturers who I've encountered work with somewhere around 1.5 percent salt. I never measure the salt. I just try to...

GROSS: What does the salt do?

KATZ: The salt pulls water out of the vegetables. The salt actually crisps up the vegetables a little bit because salt has the quality of hardening pectins. And it also - all ferments are a matter of creating a selective environment that favors certain types of organisms while discouraging other types of organisms. And lactic acid bacteria, what we're trying to cultivate in the production of sauerkraut, are very salt tolerant, while certain other bacteria that might also be present cannot tolerate salt.

And it also slows down the whole process, which really helps in terms of preservation.

GROSS: So after you shred the cabbage or whatever it is that you're fermenting, whatever vegetable it is, you have to kind of like pound it a little bit to release the juices, and what does that do?

KATZ: So you can pound it with a tool. What I usually do at a small scale is just use my clean hands and squeeze the vegetables for a couple of minutes. And what this does is it bruises the vegetables, it breaks down cell walls and basically enables the cells to give up their juices, which facilitates our being able to submerge the vegetables under liquid, which really is the point.

We're trying to protect it from the air by submerging the vegetables under liquid. And then stuff it into a jar. You want to press really hard to force out any air bubbles, and you want to make sure that the vegetables are pressed down under their juices. And then just seal the jar.

But be aware that pressure will be produced. So you don't want to leave it sealed for days and days. I like to leave it on the counter, and each morning while I'm making my tea, I just open it and release the pressure. And then after a couple of days, you can really start enjoying it.

I mean, the flavors transform very quickly. The bacteria proliferate. The texture changes, and what I recommend to people experimenting for the first time, is to just taste it at periodic intervals. You know, taste some after two days. Taste some after four days. Taste some after a week. And then you can get a sense of whether you're liking it more and more as the flavor gets more acidic or whether it's acidic enough, and you want to move it into your fermentation-slowing device, which is your refrigerator.

GROSS: So you're adding water, or you're just using the juices of the vegetable?

KATZ: I hardly ever add water. You get a much more concentrated flavor if you can just get the juices out of the vegetables. You know, if for whatever reason you can't squeeze the vegetables, if you're using very old vegetables that have sat in a warehouse, often they have, you know, evaporated some of their juices. So sometimes it's not possible, but almost always it's possible to get the juices out of the vegetables, and you'll get a much more concentrated flavor if you don't add any water.

GROSS: But you can add water if you need to?

KATZ: Yeah, and also, you know, like I described sour pickles. If you want to leave a vegetable whole or in large chunks, then it's impossible to pull the water out of the vegetables. So that's when you mix up a brine. But typically in sour pickles or other brined vegetables, you know, when you have it floating in a bunch of salt water that you're adding to it, you add lots of spices to sort of compensate for that dilution of flavor.

And you can certainly flavor this with any kind of spices. I mean, in the German sauerkraut tradition, often people used juniper berries or caraway seeds. In Russian traditions, often they'll at a little bit of fruit like cranberries or apples. In the Korean kimchi tradition, it's usually hot pepper and ginger and garlic and onions. But really you could experiment with, you know, any kind of seasonings that you like to spice it up.

GROSS: Since we're dealing with bacteria here and hopefully with the correct bacteria, if you make any kind of fermented vegetable yourself, and you eat it, what are the odds you're going to make yourself sick because it grew the wrong kind of bacteria?

KATZ: Well, according to the U.S. government, there has never been a single case of food poisoning in the United States from fermented vegetables. I mean, this food is about as safe as it gets. You couldn't say that about raw vegetables. I mean, all the time we hear about people getting sick from, you know, lettuce, spinach, tomatoes or, you know, other raw vegetables.

And, you know, the point with sauerkraut is that, you know, even if your vegetables had been subjected to some sort of incidental contamination, the indigenous populations of lactic acid bacteria, you know, are so great that, you know, once you create an environment that's hospitable for them, they easily dominate, you know, any kind of incidental contaminant that might also be present.

And then the acidification basically would destroy any pathogenic bacteria. You know, none of the pathogenic bacteria that we hear about or worry about can survive in an acidic environment. So this is actually a process that makes food safer, as well as preserves it.

GROSS: Let's talk about yogurt, which is, you know, one of the most popular fermented foods. What's the principle behind yogurt?

KATZ: The principle behind yogurt is almost the same as the principle behind sauerkraut. We are using lactic acid bacteria to preserve food. The method for it is somewhat different. There are many different types of lactic acid bacteria.

And the ones that are used in most yogurt traditions are bacteria that we would describe as thermophilic, meaning that they are most active in an elevated temperature range. So usually when you make yogurt, you want to incubate the yogurt by creating an environment that stays between 110 degrees and 115 degrees Fahrenheit.

GROSS: Do you need a starter to introduce the right bacteria to the milk that you're making the yogurt from?

KATZ: Yes, absolutely. Yogurt is the classic example of what is often called a cultured food. And the cultures are - is the community of bacteria that you're introducing, and the act of introducing it is called culturing the food. So yes, to make yogurt you always need basically a batch of mature yogurt, and that's what you introduce is just a little spoonful of mature yogurt into the fresh milk that you want to turn into yogurt.

GROSS: And then what?

KATZ: Well, OK, first what you do is you take your milk, and you heat it up. I usually heat it to about 180 degrees and then cool it down to about 115 degrees and introduce my starter culture. And then for an incubation space, usually what I do is I take an insulated cooler, and I preheat it with hot water. So it's preheating while I'm heating up the milk and cooling down the milk.

And so if I get that insulated chamber to be in the 110- to 115-degree range, then I just take my milk that I have cultured with the yogurt starter and put it into that 110- to 115-degree chamber and leave it there for somewhere between four to eight hours. And when I take the jars of milk out, they have solidified into yogurt.

GROSS: So what's the difference between curdled milk and yogurt?

KATZ: Well, OK, let me talk a little bit about milk and drinking milk. You know, the milk that we grew up with, you need a refrigerator for that. Fresh milk is really a phenomenon of the 20th century in regions of the world where refrigeration became widespread.

The milk that most people in the world have historically enjoyed and that, you know, many people in the world continue to enjoy are sour forms of milk. Yogurt could be considered a sour form of milk. Kefir could be considered a sour form of milk.

But we have a word in the English language - clabbering. Clabbered milk, and clabbered milk is essentially raw milk that is allowed to sit at ambient temperatures. And after, you know, approximately 24 hours, you know, give or take, depending upon the temperatures, that milk begins to thicken. And when it begins to thicken, you know, then we call that clabbered milk.

But the flavors of clabbered milk will be very, very different with different milks in different environments and different seasons and different temperature ranges. And so, you know, when people have a flavor in their clabbered milk that they especially like, they seek to perpetuate it by adding a little bit of that batch that they really liked into the next batch.

And this is really the story of fermented milk products all around the world. And, you know, yogurt is just the one that became, you know, the global superstar that's, you know, sold in every supermarket.

GROSS: Sandor Katz is the author of "The Art of Fermentation." In part two of our discussion, which we'll hear another day, he'll talk about cheese, meat, beer and wine. I'm Terry Gross, and this is FRESH AIR. Transcript provided by NPR, Copyright National Public Radio.